Re: when we use plural or singular form for currencies ?
I'm not sure what you mean by a rule for it. Countable nouns are plural when there is more than one of them.
One dollar = singular
200 dollars = plural
However, not all currencies take an "s" at the end to indicate the plural. Most of them do take the plural "s".
Remember the rules of written English:
- Start every sentence with a capital letter.
- End every sentence with the relevant punctuation mark (full stop, question mark, exclamation mark).
- Do not put a space before a full stop, comma, question mark or exclamation mark.
- Put a space after a full stop, comma, question mark or exclamation mark.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.